Large-size LCD panels usually use cold cathode fluorescent lamp (CCFL) as conventional backlight source. Each CCFL has two electrode terminals where hard-ware wires are connected to a power source, usually a DC/AC inverter. Due to the increased panel size and number of lamps used in large LCD panel applications, the wiring becomes a complex burden both in manufacturing and cost.
External electrode fluorescent lamp (EEFL), on the other hand, has both electrodes exposed at both ends of the lamp, does not require any hard-ware wiring. Such that EEFL is used in in large LCD panel applications. Multiple EEFLs are placed in parallel on the back of the LCD panel. There are two metal plates for electrical connections. One plate connects to one end of all the EEFLs and the other plate connects to the other end of all the EEFLs. The power source, generally a DC/AC inverter, simply provides power to both metal plates for operating the EEFLs.
FIG. 1 is a conventional EEFL driving circuit 10 plus its current sensing circuit 260 in accordance with the prior art. As shown in FIG. 1, a conventional driving means plus its current sensing circuit 260 is illustrated. Generally, the driving circuit 10 includes transformer 220 and 222, which are configured with 180 degree off phase to each other. Thus the quasi-sinusoidal waveforms generated by transformer 220 and 222 are of 180 degree phase difference. Since all the EEFLs 40 are connected in parallel by its nature, the regulation of the EEFL current is implemented by sensing and controlling the sum of the current flowing through each EEFL (EEFL(1), EEFL(2) . . . EEFL(n)). The sensing circuit 260 includes two sensing resistor 17 and 18. As known by those skilled in art, sensing circuit 260 can sensing the total current by sensing the voltages on the sensing resistor 17 and 18. One drawback occurs when one of the EEFLs 40 is not connected properly. The rest of the EEFLs will be overdriven since the controller controls the total current only. For example, if one of the EEFLs, EEFL(n) gets open, then EEFL(1) to EEFL(n−1) will be overdriven by excess current flowing through. This will degrade the life time of the EEFL.